Osteoporosis is a growing epidemic that afflicts aging men and women across the globe. Osteoporosis-related fractures cause dramatic morbidity and mortality. Current therapies fall vastly short of rehabilitating osteoporotic skeleton to halt fractures. Intermittent daily parathyroid hormone (PTH) injections are the only FDA-approved therapy that is anabolic to bone and improve bone quality. Its dosing is, however, limited to a low level and a short duration due to potential adverse effects -- hypercalcemia and induction of osteosarcoma. Our proposal aims to delineate mechanisms underlying the osteoanabolic actions and the hypercalcemic effects of PTH in order to devise new strategies to enhance PTH therapy. Because raising [Ca2+] and allosteric CaSR agonists (i.e., a calcimimetic) activate the extracellular Ca2+-sensing receptors (CaSRs) in osteoblasts (OBs) to promote their survival and differentiation and in osteoclasts (OCLs) to inhibit their survival and bone- resorbing functions, we propose to use a calcimimetic (i.e., NPS-R568) to activate CaSRs in those cells to promote osteoanabolism of PTH and prevent hypercalcemia. We hypothesize that the activation of CaSRs in the OB and OCL lineages are essential steps in producing osteoanabolic responses to intermittent PTH and that combined treatment with a calcimimetic will prevent hypercalcemia and enhance the osteoanabolic effects of intermittent PTH by simultaneously potentiating CaSR activities in OBs, and OCLs. Aim 1 will determine (i) whether activating CaSRs in OBs and OCLs by co-administration of NPS-R568, abrogates the hypercalcemic effects of PTH and promote osteoanabolism by increasing bone-forming capacities and/or prolonging the anabolic window in various mouse models of bone loss; and (ii) whether drug intermissions allow for the repopulation of osteoprogenitors and enhance osteoanabolic effects of the combined treatment. Aim 2 will determine whether the activation of CaSR in OBs is required for the osteoanabolism of the combined PTH/NPS-R568 treatment in vivo; and delineate the underlying mechanisms by examining the effects of the compounds on the differentiation and functions of OBs in vitro. Aim 3 will determine whether the activation of CaSR in OCLs is required to prevent the development of hypercalcemia and enhance osteoanabolic effects of the combined PTH/NPS-R568 treatment in vivo; and delineate the underlying mechanisms by examining the effects of the compounds on the differentiation and functions of OCLs in vitro. Our preliminary studies showed that co-injections of intermittent PTH with NPS-R568: (i) prevented hypercalcemia, (ii) enhanced anabolic effects on trabecular bone by 2-3 fold, (iii) promoted osteoanabolism and increased bone strength at cortical sites, (iv) extended the anabolic window, and (v) permitted uses of higher doses of PTH; and that knockout of CaSR gene in OBs completely ablated the anabolic effects of PTH. The successful completion of this proposal will establish a new paradigm for the anabolic actions of intermittent PTH and establish a robust regimen to restore osteoporotic skeleton.